Four birthdays in five get a different sign
There are two zodiacs in use, they are 24.23 degrees apart, and the gap grows every year. We counted the consequence: on 295 of 365 days in 2026, the two disagree about the Sun sign. The Sun today is one thing in one and something else in the other.
Anything in a violet panel we computed from DE421 ephemeris data. You can check it.
Anything on a gold rule was said by people, at a particular time. We say who, and when.
The wobble
WHAT IS ACTUALLY MOVINGThe Earth spins on a tilted axis, and that axis does not point steadily. It traces a slow cone, the way a spinning top leans and circles before it falls. One full circuit takes about twenty-six thousand years.
The consequence is that the spring equinox, the moment the Sun crosses the celestial equator going north, slides backwards against the stars a little each year. Anything measured from that point therefore slides too.
Not quoted from a table. Measured by asking the ephemeris where the two zodiacs stood in 2000 and in 2050 and dividing. The published figure is about 50.3 arcseconds a year and a cycle near 25,772 years, so the arithmetic agrees with the literature.
Which gives you two zodiacs
TROPICAL AND SIDEREALThe tropical zodiac fixes zero degrees of Aries to the spring equinox. It is a calendar of seasons: Aries begins when spring begins, by definition, and the constellations are not consulted.
The sidereal zodiac fixes its zero to the stars instead, and so follows them as the equinox drifts away.
Two thousand years ago the two nearly coincided. They have been separating ever since, at the rate above.
This offset has a name, the ayanamsa, and it has more than one definition. Ours is Lahiri, the one Indian astrology standardised on. Others differ by around a degree, so a figure quoted without naming its definition is not quite a figure.
What that does to a birthday
THE COUNTNearly a full sign of separation means that for most of the year the two systems put the Sun in different signs. Rather than reason about it, we walked through 2026 a day at a time.
The one day in five where they agree is not luck. It is the part of each sign the drift has not yet carried across the boundary.
The same Sun, the same instant, one measurement. Two answers, because they are answers to two different questions.
| Date | Tropical | Sidereal |
|---|---|---|
| 15 Jan 2026 | Capricorn 25°21′ | Capricorn 1°07′ |
| 15 Feb 2026 | Aquarius 26°48′ | Aquarius 2°35′ |
| 15 Mar 2026 | Pisces 24°54′ | Pisces 0°41′ |
| 15 Apr 2026 | Aries 25°32′ | Aries 1°18′ |
| 15 May 2026 | Taurus 24°40′ | Taurus 0°27′ |
| 15 Jun 2026 | Gemini 24°25′ | Gemini 0°11′ |
| 15 Jul 2026 | Cancer 23°02′ | Gemini 28°48′ |
| 15 Aug 2026 | Leo 22°41′ | Cancer 28°28′ |
| 15 Sep 2026 | Virgo 22°40′ | Leo 28°27′ |
| 15 Oct 2026 | Libra 22°09′ | Virgo 27°55′ |
| 15 Nov 2026 | Scorpio 23°08′ | Libra 28°54′ |
| 15 Dec 2026 | Sagittarius 23°31′ | Scorpio 29°17′ |
Find the month you were born in. Where the two columns differ, your sidereal Sun sign is the one before your tropical one.
And the beginning of the year shows the same thing as a date rather than a degree. The Sun enters tropical Aries at the equinox, and sidereal Aries 24.55 days later.
Neither zodiac is drifting away from the truth. They are measuring from two different marks, and the marks are moving apart.
The thirteenth sign, and what NASA actually said
A STORY THAT RETURNS EVERY FEW YEARSEvery so often the headline comes back: there is a thirteenth sign, your sign has changed, and NASA said so. Three separate things are being run together, and pulling them apart is straightforward.
First, the constellation is real. Ophiuchus sits across the ecliptic and the Sun does pass through it, roughly from 29 Nov 2026 to 18 Dec 2026. That much is astronomy and it is not new: the constellation is described by Aratus in the fourth century BC and by Ptolemy in the second AD.
Second, those dates are the answer. Here is where the Sun actually stands in the tropical zodiac on them.
Both inside one existing sign. The constellation overlaps a stretch of sky the tropical zodiac had already named, so there is nothing to add and nothing to renumber.
The reason is definitional rather than a matter of opinion. Constellations are patches of sky of unequal size whose modern borders were drawn by Eugene Delporte and adopted by the International Astronomical Union in 1928, published in 1930. Signs are twelve equal thirty-degree divisions measured from the equinox. They have not been the same thing for two thousand years, and the tropical zodiac does not consult constellations at all.
Third, NASA. The recurring story traces to a Space Place post aimed at children, and the agency's own words are unambiguous: they did not change any zodiac signs, they did the arithmetic, and they noted that the Babylonians themselves left Ophiuchus out to keep the count at twelve. NASA was not commenting on astrology. It was explaining a difference between constellations and signs, which is the same distinction this section is making.
Who noticed, and how
THE INTERPRETATION, AND ITS HISTORYFrom here down we are not measuring. We are telling a story, with names and dates.
Hipparchus is credited with noticing the drift, and the way he did it is worth knowing because it is the same method as the count above. He measured the longitude of the bright star Spica, compared his figure with observations made by Timocharis and Aristillus about a century and a half earlier, and found the star had moved two degrees relative to the equinox.
He was careful about what he could conclude, which is more than most of his successors were. His estimate was that precession was not less than one degree a century, an upper bound on the cycle rather than a value: no more than thirty-six thousand years. The real figure is roughly 25769, so he was right about the phenomenon and deliberately loose about the number. Do not let anyone tell you he calculated the modern rate.
Because the two zodiacs coincided when the tradition was being written down, the choice between them did not matter at the time, and different traditions ended up on different sides of it without a quarrel.
Western astrology kept the equinox and so is tropical. Indian astrology kept the stars and so is sidereal, and standardised on the Lahiri ayanamsa, the one used here. Both have been internally consistent for centuries. What neither can claim is that the other is simply measuring badly.
Which is the useful thing to take from all of this. The drift is not an error anybody made, and it is not a scandal waiting to be exposed. It is a fact about the Earth's axis, it has been known for twenty-two centuries, and the only question it raises is which mark you would like to measure from. That is a question with two defensible answers and no secret one.
How these numbers were arrived at
METHOD, AND ITS CHECKThe Sun's positions come from NASA's DE421 ephemeris data, by the same machinery that computes your birth chart inside the app. The offset between the two zodiacs comes from Swiss Ephemeris, an independent implementation, using the Lahiri definition.
The rate was not copied from anywhere. It is the change in that offset between 2000 and 2050 divided by the time, which is why the article can put a number on it and also say what it agrees with. The disagreement count is a walk through every day of 2026, comparing the sign in one system with the sign in the other.
One caution the arithmetic cannot remove. The gap depends on which ayanamsa you pick, and the common definitions differ by about a degree, which is enough to move a birthday across a boundary. Any figure of this kind that does not name its definition is incomplete, including ours if we had not named it.
Sources
SO YOU CAN CHECK THEM YOURSELF- Positions: JPL DE421. Ayanamsa and sidereal longitudes from Swiss Ephemeris 2.10.03, Lahiri definition.
- Hipparchus, 2nd century BC, comparing Spica against Timocharis and Aristillus; his estimate was a bound of not less than one degree per century.
- The 88 constellation boundaries, drawn by Eugene Delporte, adopted by the International Astronomical Union in 1928 and published in 1930.
- Ophiuchus described by Aratus, 4th century BC, and Ptolemy, 2nd century AD.
- NASA Space Place, 2016, the origin of the recurring story, stating that no zodiac signs were changed. Note the original page has since moved.
- A note on the history: the astronomy above is measured and checkable. The historical attributions rest on encyclopaedic sources rather than peer-reviewed history of science.